3.2.2 Group two, the alkaline earth metals

Trend in atomic radius

Increases down the group

Atoms have more electron shells making the atom larger, sheilding increases

So weaker attraction between nucleus and outer electrons

Trend in first ionisation energy

Decreases down the group

More electron shells

Sheilding increases

The ions get bigger so weaker attraction between positive nucleus and electron being lost

Less energy required to remove outer electron

Trend in melting points

Decreases down the group

Number of electron shells increases so shielding increases

Positive ion becomes bigger

Each atom contributes two electrons to electron sea

Distance between the positive ion and delocalised electron increases

weaker attraction between the positive ions and delocalised electrons, so metallic bonding becomes less strong

Trend in reactivity

Increases down the group

Number of shells increase, so shielding increases

Less attraction between nucleus and outer electron as there is a greater distance between them

So electrons are more easily lost

Trend in solubility of metal hydroxides

Increases down the group

This means more OH minus ions are released to react with H plus ions in a neutralisation reaction

More OH minus ions released, making a more alkaline solution, so pH decreases down the group

This makes magnesium hydroxide and calcium hydroxide the most insoluble, so its observation when reacting with water is that it becomes cloudy due to low solubility and reacts very slowly

Barium hydroxide is the more soluble, so reacts with water rapidly, with explosive fizzing

Observations of metals with water

Mg: very slow reaction, few bubbles on surface, end mixture is cloudy

Ca: steady fizzing, slight cloudiness appears

Sr: rapid fizzing, metal sinks then rises as bubbles form

Ba: explosive fizzing, metal may float on bubble layer

Reaction of group two metals with cold water

Forms a metal hydroxide (aqueous) and hydrogen gas

Reaction of group two metals with steam

Forms metal oxide (solid) and hydrogen gas

Use of magnesium hydroxide

As an antacid- a base substance with neutralises stomach acid

Reacts with hydrochloric acid in the stomach, neutralising the excess acid that causes symptoms of indigestion

The low solubility of magnesium is an advantage as it presents the stomach pH from rising too quickly

Use of calcium hydroxide

Use for soil treatment in agriculture

Is spread on acidic soils to raise its pH

This neutralises soil acids that prevent proper plant growth

Improves nutrient uptake by crop plants

Reduces toxic aluminium levels in the soil

Use of calcium oxide and calcium carbonate

Used in sulfur dioxide removal

Sulfur dioxide is an atmospheric pollutants produced by power stations which causes acid rain

Can be removed from flu gas emissions by reacting with calcium carbonate or oxide which reacts with SO2 to form a solid waste product, calcium sulfite

Use of magnesium

In titanium extraction to extract it from its ore

The rice is first converted to titanium chloride using chlorine gas and carbon

After purifiying it through fractional distillation, magnesium acts as a reducing agents in this high temperature reaction

Essentially, the process undergoes two displacement reactions

Magnesium acts as a reducing agent as it donated electrons to titanium, and is hence oxidised

Trend in group two sulfates solubility

Solubility decreases down the group

This makes magnesium sulfate the most soluble

Barium sulfate is insoluble

Testing for sulfate ions with acidified barium chloride

Reagent: Barium chloride solution acidified with hydrochloric acid

Observation: white precipitate forms

Why is HCl needed when testing for the presence of sulfate ions

HCl is needed to react with carbonate impurities that would also form a which precipitate and give a false positive result

Use of BaSO4

Used in medical imaging as a barium meal

Barium sulfate is swallowed by patients to highlight and image soft tissues like the digestive tracts

Because barium sulfate absorbs x rays, the organs which it coats becomes visible x ray images, helping diagnose conditions

Even though barium compounds are toxic, it is safe because it is insoluble